By Edgar Mangwende, A. Ag. Plant Health Technologist.
Bacterial leaf streak (BLS) disease poses a growing threat to cereal crops in Canada, despite its historical presence in the region for a long time. There has been a concerning resurgence of this seed-borne disease in recent years, primarily attributed to an upsurge in severe weather events and the accelerated transmission via infected seeds. Although reports indicate that BLS can potentially slash yields by up to 50 per cent, such losses have not yet been observed in Western Canada. Managing this insidious disease proves challenging due to the absence of effective and economically viable in-season management tools, leaving producers with limited recourse to combat this burgeoning threat.
(Photo credit: Dr. Constanza Fleitas)
BLS is caused by Xanthomonas translucens, a bacterial pathogen responsible for both this and black chaff diseases in cereal crops. Symptoms typically start on the middle or apex of the leaf, where dew stays the longest. Early leaf infections appear as small, oval, water-soaked lesions running parallel to the leaf veins and may have a translucent appearance. If humidity is high, you might also see a “bacterial ooze” on the lesions, which appear as little yellow milky droplets visible to the naked eye. It is important to note that the presence of these exudates is not always consistent, making precise diagnosis a challenging endeavour. As the disease progresses, the lesions merge, forming larger irregular streaks that hinders photosynthesis and potentially cause withering and death of leaves, starting from the leaf tip. Accurate visual identification of the disease is difficult as it doesn’t usually occur in isolation.
To effectively mitigate the risks associated with BLS, producers are strongly advised to conduct a comprehensive assessment of the disease's potential impact on their seed lot and fields. The bacteria do not only survive the winter on crop residues in some areas, but also persist in perennial weeds, winter wheat and fall rye. Implementing an extended crop rotation with intervals greater than two years between cereal crops, proves instrumental in diminishing the presence of viable inoculum within decomposing crop debris. While inoculum from crop residues constitutes a relatively minor source of bacterial streak, the highest risk of transmission lies in farm-saved seed. It is essential for producers to remain vigilant when replanting seed from previously affected fields as BLS follows a polycyclic pattern, meaning it can undergo multiple infection cycles during the growing season. Even a small number of bacterial cells present on the seed may be vertically transmitted to leaf tissues and rapidly multiply and wreak havoc on crops.
Given the bacterial nature of the disease, conventional foliar or seed treatment fungicides do not offer effective control measures. Consequently, producers who save their own seed must exercise utmost caution when dealing with fields potentially harbouring BLS. They may opt to send seed samples to designated laboratories for testing. However, it is crucial to note that extensive research is still underway to fully comprehend the implications and thresholds associated with using seeds that test positive for the disease. Therefore, producers are strongly encouraged to commence their cultivation endeavours with certified, disease-free seeds.
The increase in adopting irrigation practices in Western Canada, coupled with an upsurge in the frequency of large storms witnessed in recent years, further adds complexity to the management of BLS. The interplay of rain splashing, wind-driven precipitation, irrigation activities and mechanical operations during irrigation intensifies the growth and dissemination of the disease. Although producers can meet the water demands of their crops through irrigation, further strategic measures can be employed to mitigate the duration of wetness on the crop canopy. Optimal practices include irrigating in the evening when the canopy is already moist with dew or allowing sufficient intervals between irrigation events to facilitate canopy drying.
Proactive measures are crucial to protect fields. Regular field scouting is essential for early detection, particularly during in-crop herbicide applications. Avoid scouting in wet conditions to prevent inadvertent spread. BLS can overwinter in crop residue, survive in volunteer plants and even hide in weedy grasses and headlands. Insects may also play a role as a vector for the bacteria to enter plants. Since bactericides are not viable options for disease control due to regulatory concerns, vigilance through scouting and managing pests and weeds in a timely manner can enhance crop health and resilience against BLS.
Varietal resistance is crucial for controlling bacterial streak disease. Historically, bacterial diseases haven't caused significant yield loss in Western Canada. Consequently, limited breeding efforts have been made for BLS resistance and the number of varieties resistant to BLS or black chaff remains unknown. In areas with higher incidence of BLS, screening for resistance has been ongoing for the past 30 years, including recent research in the Northern Great Plains of the United States. Certain varieties, such as Glenn, Faller, Prosper and Bolles, have shown disease tolerance. Canadian barley and wheat research projects aim to identify sources of resistance for incorporation into commercial lines.
While bacterial streak disease poses a significant threat to cereal crops, producers may mitigate the consequences of this disease by remaining vigilant, conducting thorough risk assessments and adopting proactive management practices and preventive measures.
If you suspect BLS in your crop, send your samples for analysis and diagnosis to the Provincial Crop Protection Lab. at 1610 Park Street, Regina, SK S4N 2G1.